The desirability of a mixture of EC and PC as plasticizers to increase the conductivity of alkali metal ion-conductive solid polymer electrolytes has been clearly taught in U.S. Pat. Nos. 5,219,679 and 5,252,413 which disclose poly(acrylonitrile)- and poly(vinyl chloride)-based electrolytes. The use of the plasticizer mixture of EC and PC results in high conductivities in the ambient temperature range of -20.degree. to 50.degree. C.; for example, typical conductivities of 10.sup.-4 ohm.sup.-1.cm.sup.-1 at -20.degree. C., 2.times.10.sup.-3 ohm.sup.-1.cm.sup.-1 at 25.degree. C. and 4.times.10.sup.-3 ohm.sup.-1.cm.sup.-1 at 50.degree. C. have been realized. When these solid polymer electrolytes are used in solid-state Li-ion batteries containing a graphite composite anode, the PC undergoes electrochemical reduction on the graphite electrode generating gases during the initial charge by which the battery is activated. Because of this reaction of PC, the battery cannot be discharged after the initial charge. Thus, while the presence of PC yields an electrolyte with high conductivity, it prevents the Li-ion battery from being rechargeable. Consequently, a method to mitigate or eliminate the reduction of PC is highly desirable to provide rechargeability to the battery.
Wilkinson et al. (U.S. Pat No. 5,130,211) also observed electrochemical reduction of PC on graphite when the latter was used as the anode in liquid electrolyte-containing cells in which the liquid electrolyte was composed of a solution of a Li salt in PC. They claimed that addition of one or more compounds selected from the group consisting of glymes, crown ethers and cryptands to the liquid electrolyte mitigated the reduction of PC on graphite. However, Wilkinson et al. did not teach a method of providing rechargeability to solid polymer electrolyte-based Li ion batteries containing graphite anodes, nor were they specific about the most effective additive for use in solid-state batteries. We have found that a convenient way of alleviating the reduction of PC in solid-state batteries and, thereby, providing rechargeability to such batteries is to incorporate 12-crown-4 ether in the composite graphite anode. We have also found that the most important criterion for the use of 12-crown-4 ether as an effective additive in said solid-state batteries is the mass ratio between 12-crown-4 and graphite in the composite anode. An optimum amount of 12-crown- 4 ether must be present in the composite anode. Accordingly, a principal object of this invention is the provision of a method for preventing the reduction of PC in solid-state batteries containing graphite anodes, thereby, providing rechargeability to such batteries by means of a graphite composite anode containing an optimum amount of 12-crown-4.